Revise unit factory

[mbe24]

Add some kind of register method in order to have all the units register to the factory.
In this way adding a new unit beomes as simple as implementing the unit, defining a unit id and registering this id to the factory within the class that implements the unit.

[mbe24]

Problem hereby is that units would only register themselves if the correspoding implementing classes get loaded. So other mechanism is needed.